Over the first three years of grant support, our laboratory has been instrumental in showing that many of the basic elements of the immune response are present in Alzheimer's disease (AD) brain (1-4,7-14). Much of our data has now been replicated by outside laboratories (15-26). Fundamental questions remain, however. In particular, we need to know how immune processes arise in the AD brain, and how significant they may be to AD pathogenesis. Research with complement cascade proteins may provide one of the most direct approaches to answering such questions. This is because the complement proteins mediate the business end of the immune response: cell lysis and inflammation. Clq, C3, C4, C5, C6, and C9 are not detected in nondemented elderly (ND) brain, but are profusely and specifically immunoreactive in AD brain in association with neuritic plaques, neurofibrillary tangles, and some neurons (2,3). Because the completed complement cascade results in tissue lysis, and can provoke reactions in nearby autologous tissues as well, complement activation is likely to have pathogenetic significance in AD. The specific aims of the present proposal therefore focus on the source, target, induction, and regulation of complement in AD-brain. Specific Aim 1: In situ hybridization experiments will seek to determine whether the Clq and C9 observed in AD brain has an endogenous or peripheral source. Peripheral macrophages are the most important extrahepatic source of complement (30), and our research has shown that brain microglia are immunopositive for many of the same immune factors as peripheral macrophages (2-4,11-14). Specific Aim 2: Single and double label immunohistochemistry and immunoelectron microscopy will be used to evaluate cellular and subcellular targets of Clq, C4d, C5, and C9. Targets of particular interest are molecules associated with AD pathology such as tau, Alz 50, and the amyloid precursor protein (APP). Specific Aim 3: A novel Western blot approach employing preincubation with purified Clq followed by anti-Clq detection will probe for I g and non-Ig Clq reactive ligands. Such non-Ig ligands are known to exist, can generate antibody independent complement activation and immune pathology (27,31-33), and should be sought given the lack of conclusive evidence for AD specific Igs (10,25,63). Specific Aim 4: We will employ Western blot techniques in order to determine whether regulation of complement activation by Cl inhibitor and Factor I is deranged in AD. Our laboratory has demonstrable expertise with the proposed techniques. A better understanding of complement activation in AD brain would lend increased insight into mechanisms of AD pathogenesis and might even lead to useful therapies for AD based on anti-inflammatory drugs.